The term “multimedia” denotes in the broadest sense all applications in which expediently supplementary voice, audio, text, still image and moving picture communication are integrated. A narrower definition of the term “multimedia” stipulates that at least three different contents such as, for example, text, image or sound have to be used in an application or a so-called multimedia service. Such multimedia services are e.g. Internet services such as, for example, the World Wide Web, etc. or video or audio streaming, in which video or audio data are transmitted continuously.
In addition to the ever more widespread use of the Internet—in Germany, for example, the Internet was used by 60% of the population in 2004 according to the report “Information technology in companies and households 2004” by the Federal Office of Statistics—the increasing prevalence of multimedia services in telecommunications is regarded as a driving force behind the introduction of new technologies which provide greater bandwidths for access to a telecommunication network and for the transmission of data to a subscriber.
For a line-connected network access—that is a subscriber is fixedly connected to the telecommunication network via an access line—e.g. xDSL (Digital Subscriber Line) technologies, cable modems, fiber-optic technology, etc. enable access with greater bandwidth on the subscriber's access line—a so-called broadband access—to a telecommunication network and hence the use of multimedia services with good quality.
For a wireless network access via the so-called “air interface”, technologies such as e.g. universal mobile telecommunications system UMTS or the further development of UMTS, the so-called next generation network NGN in mobile radio, which is also referred to as the next generation mobile radio network, or wireless LAN are likewise offered, which are optimized for higher data transmission rates. These technologies enable the use of multimedia services such as e.g. video telephony or video streaming or access to the Internet with good quality even from mobile terminals such as e.g. mobile telephones or laptops.
Since multimedia services presuppose the multimedia data such as, for example, texts, graphics, audio or video sequences in digital form, so-called packet-switching telecommunication networks are used for transmitting said data to the access lines of the subscribers; in said telecommunication networks, a permanent, continuous channel is not made available for a connection, rather the data are transmitted in the form of packets.
Protocols are used for controlling the data communication within a packet-switching telecommunication network to the subscribers. For the transmission of data but also for the use of multimedia services, the Internet protocol IP is often used in packet-switching telecommunication networks. Packet-switching telecommunication networks which use the Internet protocol are therefore also often referred to as IP-based telecommunication networks. The best known IP-based telecommunication network is e.g. the Internet, which is the name given to the totality of all the interconnected telecommunication networks using the Internet protocol IP. In the meantime, IP-based telecommunication networks and hence the Internet protocol are also being used for the transmission of voice—for so-called IP telephony or voice-over-IP. In contrast to conventional telephony, in IP telephony the voice information is not transmitted via a switched circuit in a telecommunication network, but rather divided into data packets by the Internet protocol IP. In this case, the route through the IP-based telecommunication network, normally the Internet, is not defined.
Since the Internet in its present-day form does not guarantee an assured transmission quality between subscribers, transmission losses and dropouts can occur during IP telephony over the Internet. However, for example by prioritizing voice data packets—that is to say that these data packets are given greater preference during transmission than e.g. downloading of data from a data server or retrieval of an Internet page—it is possible to obtain a quality of service which is defined for direct communication and which permits only small delays. The quality of service—QoS—in telecommunication networks is defined and measured by means of somewhat different parameters depending on the communication standard (e.g. IP, UMTS, etc.).
In addition to IP telephony, however, some other multimedia services such as e.g. video or audio streaming, video telephony also presuppose a delay-free and continuous data transmission and a high quality of service. Therefore, a minimum bandwidth is required for the use of said multimedia services and IP telephony with good quality. If said minimum bandwidth is not available, said multimedia services can only be used with very poor quality or cannot be used at all.
If for a subscriber, then, for example when using a multimedia service such as video streaming, the bandwidth of his network access is almost completely occupied by the data transmission, further incoming multimedia calls e.g. based on IP or video telephony are rejected owing to a lack of available bandwidth. A resource management present in the packet-switching telecommunication network recognizes that the available bandwidth is insufficient for a multimedia call. Therefore, a called subscriber cannot be reached by a calling subscriber nor is he informed of an attempt to set up a multimedia call to him.
However, telecommunication networks in which the so-called time division multiplex (TDM) method is used contain functions such as “answering machine in the network” or “signaling of incoming calls via Internet protocol IP” for example, which are used while the connection to the telecommunication network is being used by the subscriber for access to the Internet.
In a packet-switching telecommunication network, in particular in IP-based telecommunication networks, however, there is e.g. only the possibility of using IP-based so-called voice mail systems, which are present centrally in the telecommunication network for recording voice messages, if the called subscriber cannot be reached. These systems then communicate a voice message by data transmission to the called subscriber. However, this procedure has the disadvantage that the voice message either has to be listened to with the quality of service or minimum bandwidth required for a delay-free and continuous data transmission or has to be transmitted to a dedicated PC-like terminal in order for it to be listened to. Therefore, these voice messages can be listened to only if the corresponding bandwidth is available to the called subscriber and is not occupied for example by the use of a multimedia service.